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Advice on lathe Threading tools

Looking for some advice on tooling for lathe threading

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old mart26/05/2020 16:46:53
1795 forum posts
138 photos

The joint between the handle and the shaft needs to be strong because of the force needed to cut these large threads.

Jim Beagley26/05/2020 23:11:25
77 forum posts
41 photos
Posted by old mart on 26/05/2020 16:46:53:

The joint between the handle and the shaft needs to be strong because of the force needed to cut these large threads.

I’ve not finalised the handle arrangement yet but I’ll take note of your comment.
The tube has a bush at the handle end that will probably get welded in somehow and attached to the handle.

Got the threading tools today so looking forward to making a mess of the floor

Jim Beagley27/05/2020 21:18:22
77 forum posts
41 photos

Well the Boxford backplate has arrived and it fits in my 4-jaw.
All dialled in, plus I’ve got the BSW insert.
So I’m guess the first step is to completely bore out to the ID to start the thread from, and the afterwards counterbore the boss for the spindle clearance?

jeez this is nerve wracking!

fbdd15a3-fcd1-48e7-97ab-c3b3144ce18c.jpeg

Jim Beagley28/05/2020 20:39:41
77 forum posts
41 photos
Posted by old mart on 19/05/2020 21:25:27:

I expect a damaged tap with a 8 tpi thread would be as rare as hens teeth. I have made a sketch of the thread which shows the main dimensions. The starting bore of the thread is not critical, it could range between 1.59" and 1.614". I have always gone for the smaller size using carbide inserts. The size is right when the first sign of the thread shows in the 1.75" register. It would take multiple passes from just touching the bore to be threaded. The register size needs to be no less than 0.001" greater than the measured spindle size, or it will be very difficult to screw the plate on.

I would go 12,10,10,8,8,7,6,5,4,3,2, and 1 in thousandths of an inch, keeping an eye out when close to the end for the witness mark on the register.

_igp2595.jpg

Edited By old mart on 19/05/2020 21:31:08

Hey Old Mart.
I have started with this process and bored the plate to 1.59D.
How critical is your measurement shown of the 0.55 depth of the 1.75D counterbore?
I have measured the plates I have, and the original S&B one is deeper than that by some amount.

Can I simply cut it to 0.55 then at the end of the procedure just extend the counterbore till the plate screws up to the face of the spindle?

cheers, Jim

Martin Connelly28/05/2020 21:29:10
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1370 forum posts
159 photos

You could screw cut the whole thread first then counterbore but that just means more thread to cut. I think the best thing to do is match what you have already since you know it works.

Martin C

Jim Beagley28/05/2020 21:34:48
77 forum posts
41 photos

Well I’ve cut the bore and counterbore to Old Marts numbers. I’m strictly metric so there was some calculator action going on!

Tomorrow I’m going to attempt the threading

Boring Complete

Jim Beagley29/05/2020 21:16:59
77 forum posts
41 photos

b37e7594-db83-4623-afe9-ecc72dcdf81c.jpegSuccess. That’s all for now, but it wasn’t plain sailing so I will have some questions in the morning.
but mainly, it’s done! Yay

5907262b-796a-46ec-a530-0f1331f81802.jpeg

old mart30/05/2020 15:05:22
1795 forum posts
138 photos

Now you have it screwed on to the spindle, it needs bedding in a bit. Make sure every bit of swarf is removed from the internal thread using a small wire brush. Also deburr the ends of the thread, a Dremel comes in useful here if you have one. Clean and oil the threads and with lowest back gear engaged, screw the plate on and off several times, fairly tight. Use a strap wrench to start it unscrewing. The plate must come into contact with the face of the spindle, if it gets tight before contact is made, you will have to cut back the threads a little adjacent to the 1.75 register in the plate.

A light facing cut or two will ensure the plate is true to the spindle axis.

Edited By old mart on 30/05/2020 15:09:19

Jim Beagley01/06/2020 18:34:17
77 forum posts
41 photos

So I wonder if I can air some of the problems I had to see if I can find out where I went wrong.

When the thread started to show on the counter-bore, the thread cut registered on the outermost edge of the boss (nearest the spindle in fitment) but didn't run along the full length of the counter-bore, which at the time I didn't understand.

I stopped at this point and reversed the plate and chuck, and the thread would start but go hardly any distance onto the spindle, so I reset, cut the thread a thou deeper and tried the fit again.
This went on for ages, with my trying to get the thread to show the full length of the counter-bore, and trying the plate on the spindle. With every pass, the thread would engage very slightly more, but nowhere near fully.

Eventually, I gave up this method and took a tiny cut off the counter-bore face and the plate screwed on instantly.

So but conclusion after all that is that maybe the counter-bore was slightly tapered?
Now I used the compound slide to machine the counter-bore, as its got a dial, whereas the carriage doesnt.
Is this a boo boo, as its the only way I can think that I could have got the taper.
In hindsight I realise I could have used the DTI to get the depth of cut from the carriage.

So, sorry for the long post, but its complicated (to me anyhow).
Anyone got any thoughts on that lot?
I've got another Boxford 1-1/2 plate to have another go but it'd be better to get it less wrong 2nd time around.

Cheers,
Jim

 

Edited By Jim Beagley on 01/06/2020 18:36:03

Edited By Jim Beagley on 01/06/2020 18:36:40

Edited By Jim Beagley on 01/06/2020 18:37:00

Edited By Jim Beagley on 01/06/2020 18:37:54

Martin Connelly01/06/2020 19:16:08
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1370 forum posts
159 photos

You should have used the carriage to cut the register without a taper. You can use the compound slide to set the depth and use a stop for the carriage to do this. Some Smart and Brown lathes have a positionable stop that fits between the ways, there is a small hard point on the leading and trailing faces of the carriage that contacts it. If you don't have one then a simple stop can be made to grip the nearside dovetail to act as a carriage stop.

Position the stop, move the carriage up to the stop. Set the compound slide dials to zero with the tool just contacting the face of the workpiece. Wind the carriage away from the work piece and advance the compound the required depth.

Now when the carriage is taken up to the stop the tool goes the correct distance into the bore.

You can use the compound to do the job but it must be set to exactly parallel to the ways using a dti and suitable test bar.

Slight errors in the compound angle away from perfectly parallel to the ways will not cause issues setting the depth (cos of a small angle is very close to 1) but will cause tapering of the bore produced.

Martin C

p1150201.jpg

The Model M has a clutch so you can power up to the stop without damaging the lathe.

Edited By Martin Connelly on 01/06/2020 19:27:21

Jim Beagley01/06/2020 20:42:22
77 forum posts
41 photos

Hi Martin.
That all makes perfect sense.

I have a stop and did exactly as you suggested and now have a perfect counterbore using the carriage.
Its so so simple when someone tells you what to do but it seems insurmountable beforehand.
My apron has a clutch but I have not used it except to adjust it to engage the apron or cross slide.
I seem to have a slight fear of breaking it!

OK so now to cut the thread again and see how it works this time.
I really do appreciate all the time you guys are taking to answer my questions.

Thanks

old mart01/06/2020 21:39:34
1795 forum posts
138 photos

Don't worry, the plate will be perfectly ok. You now know that lining up the degrees to zero on the compound doesn't mean much. It is possible to get the compound parallel to the spindle axis, but its a lot of bother.

I know that your plate will be perfectly usable as we have a large quantity of faceplates and chucks at the museum. The supposedly best backplate I made has a register with 0.0005" clearance on the spindle. It is a pain to screw on and off. The loosest one has at least 0.020" clearance, it is easy to fit. They both repeat perfectly each time they are fitted. Remember what I said about screwing the plate on and off a few times before final machining of the front face.

The next time you make a backplate, you will make one with greater accuracy, but in the real world, it won't work any better than your Mk 1.

This is one of the rare times that you can get away with these errors, most other threading jobs will not be so forgiving.

Jim Beagley01/06/2020 22:19:48
77 forum posts
41 photos

Thanks Old Mart; that’s good to hear.

I am both learning how to use the machine and learning how to do a task. But it’s good learning especially getting good advice.
I was wondering if it was possible to replace the scale on my cross slide as it’s barely legible, but maybe it’s not worth it?

i only have a set or digital verniers at the moment. Is it worth me getting some internal gauges and a micrometer?
My immediate goal is the make a Stuart S50.

Jim

PS- which museum?

Martin Connelly01/06/2020 23:08:04
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1370 forum posts
159 photos

I don't use digital calipers (they are not verniers), I tried but could not get sufficiently repeatable readings because every time I tried to hold the reading or clamp the screw they moved and trying to read them in situ was hard work. I have a couple of mechanical vernier calipers with a coarse clamp screw and thumbwheel like the type shown in this link.

Workshop Technology Blog

A micrometer is useful for as well and allows you to keep the calipers for general use and keep the micrometer in good condition for when accurate jobs require it.

Bore gauges are probably something that can wait. Usual method for model engines is bore a parallel, round and smooth cylinder and then machine the piston to match the cylinder. The actual diameter is not usually critical. You can make go/no go gauges for a lot of jobs if you have a good micrometer.

Martin C

old mart02/06/2020 15:03:42
1795 forum posts
138 photos

I have several digital calipers, three cheap Chinese 150mm/6" size and my old 150mm/6" Mitutoyo and the heavyweight 300mm/12" Mitutoyos. I can trust the Mits to measure exactly, inside or out, but not the cheapies, especially not inside.

A set of tee shaped rocker gauges is worth getting, and getting to know well. For bores, if you don't have the means to measure inside accurately, the safe way is to make a plug gauge. Say, if your bore was to be 50mm, then a double ended plug gauge with one end 50mm and the other 49.95mm would work quite well. When the small end fits, you are close and the full size should not quite go in. Size for size will not fit without force.

It is worth gradually building up a few good make second hand micrometers, they are usually easy to judge from their cosmetic condition whether they have been abused.

Jim Beagley02/06/2020 16:22:00
77 forum posts
41 photos

That's all useful information - thanks.
My digital calipers (you're quite correct that they aren't verniers of course Martin) are actually Mitutoyos and I'm quite happy with them.
I will look at getting some second hand mics as suggested. M&W I guess is the go to, but are there any other recommended makes?
I'll get some of the cheap internal gauges and see how I get on with them.

Tony Pratt 102/06/2020 16:33:14
1130 forum posts
5 photos

Mitutoyo do some very nice tools, but if buying new get from a proper tool dealer [RS/Cromwell etc.], beware of counterfeits.

Tony

Martin Connelly03/06/2020 09:23:36
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1370 forum posts
159 photos

Jim, the Model M Mk2 has a cone and cup clutch in the apron to engage the drive to the carriage or cross slide. It also has a torque limiting clutch on the carriage to limit the drive as well. It was the torque limiting clutch I was referring to for driving up to a stop. I don't think the Sabel has this torque limiter, it is a spring loaded ball in pocket system and makes a loud noise when it operates. If you look at pictures of the Mk2 apron it sticks out of the side and the drive shaft passes through it.

Martin C

old mart03/06/2020 14:01:24
1795 forum posts
138 photos

Before buying mechanical micrometers, you need to make up your mind whether to choose imperial or metric. I have both types and have no bother which I use, the lathe and the Tom Senior are imperial, but the drill mill is metric. That is only a suggestion, as converting one to the other is easy, you can use your digital calipers as a converter any time. The last time I bought a micrometer, I got a bargain, a NOS 25-50mm with a length bar in a nice box with instructions, quite old as it is made in the DDR.

Jim Beagley03/06/2020 14:38:31
77 forum posts
41 photos

Once again, all good advice.
Mart, most of the lathe is imperial, although it now has a metric cross slide.
I suppose metric is more natural to me so that’s probably what I’ll get.
The verniers I have seen seem to do both as well.

I was lucky enough to find a 100T/127T Boxford gear wheel which fits the Sabel perfectly and I also now have a 36 and 40T stud gears so I can cut some metric threads!

Can anyone broach a key way into a spacer for me if I send the spacer? I need one to move the 56T screw gear in and out to meet the metric conversion gear and it’s missing.

i also just picked up another well abused Sabel from eBay which has an original cabinet and some other parts missing from my machine which is great news. I’m currently removing 70years of grime from the cabinet in prep of paint.

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